Electronic fingerprint sensing has received increased attention as a technique for reliable identification of individuals. Electronic fingerprint sensing may be used in stationary equipment, such as security checkpoints, and in portable devices, such as mobile phones and other wireless devices, and smart cards. Accordingly, electronically fingerprint sensing systems are required to be compact, highly reliable and low in cost.
Various electronic fingerprint sensing methods have been proposed. Known methods include optical sensing and capacitive sensing with a two-dimensional array of electrodes.
Capacitive fingerprint sensing using a swiped finger technique is disclosed in the International Publication No. WO 02/47018, published Jun. 13, 2002. Conductive elements, or plates, are formed on an insulating substrate to create a one-dimensional capacitive sensing array for detecting topographic variations in an object, such as a finger. The array includes multiple drive plates which are sequentially excited with short duration electronic waveform bursts. An orthogonal pickup plate connected to a charge sensing circuit sequentially detects the intensity of the electric field created by each drive element. With each complete scan of the drive plates, a one-dimensional slice of the fingerprint is acquired. By swiping a finger across the gap between the drive plates and the pickup plate, and scanning the gap at a much faster rate than the swipe speed, a two-dimensional image based on capacitance is generated. The image represents the fingerprint.
In order to combine the multiple one-dimensional image slices into a two-dimensional fingerprint image, finger swipe speed is required. The two-dimensional fingerprint image can be formed using the sensed swipe speed to control how the one-dimensional image slices are combined. Finger speed can be determined by sensing finger position at different times during a swipe over the image sensor. Various finger speed and finger position detectors have been proposed in the prior art. For example, U.S. Pat. No. 6,002,815, issued Dec. 14, 1999 to Immega et al., discloses a method and optical apparatus for capturing an image of a moving object using multiple linear arrays wherein the finger swipe speed is estimated from a set of arrival times for fingerprint minutia at successive arrays using template-matching correlation techniques. Prior art devices such as that mentioned above suffer from finger dependent correlation performance in the tracking algorithm. Furthermore, all optical techniques suffer from the ease at which they can be spoofed using relatively inexpensive artificial fingers and finger coverings.
Consequently, there is a need for improved finger position sensing apparatus and methods that provide consistently accurate finger position and speed measurements simultaneous with secondary biometric information to drastically reduce the probability of spoofing.